Indium-gold article and method



Patented Apr. 1948- u -irEo STATE INDIUM-GOLD an'rwwann METHOD Harold ,n. Ellsworth, Clinton, N. 1., assignor' to The Indium Corporation of America, New York, I N. Y., a corporation of New York No Drawing. Application May 21, 1943, Serial No. 4:41.931

dium in gold alloy. the metal soonbecoines so brittle as to be unworkable by ordinary methods and of only limited usefulness.

I have now discovered that these objectionable properties of the high indium gold alloys disappear when they exist in thin surface layers supported by an integral backing of a tougher metal, whereas the high indium alloys, even in a very thin surface zone, still exhibit highly desirable physical properties and are highly resistant to corrosion. In the copending application, Serial No. 487,932, filed herewith, now abandoned, I have disclosed the surface coating by various methods with fused gold-indium solders.

I have now discovered that a superior surface can be developed on articles of gold, rolled gold and gold plate by applying to the surface a thin coat of indium and causing it to 'alloy with the surface gold until a desired high indium content gold alloy is developed at the surface of the article. By'this means I am able to take advantage of desirable properties of these high indium gold alloys even in articles formed so as to require mechanical working of metals of which they are composed; and Tom also take advantage of desirable physical characteristics developed in the gold by mechanical working, in alloys which would not ordinarily be capable of such working.

According to my invention, I take advantage of the property of indium of diffusing readily into another metal at a'melting point above that of pure indium, but below that at which the other metal fuses or at which the indium alloy of the other metal would fuse. The indium must be brought into intimate surface relation with the other metal into whichit is to be diffused, and this I achieve according to my present invention, advantageously by electroplating one of the metals onto the other, or by exposing the solid metal to fused indium, e. g., a gold article may be hotdipped into a bath of fused indium. After this intimate surface contact is attained the solid metal article is heated to a temperature above the melting point of the indium for a period sufficient to cause diffusion of the indium into the solid metal to such extent that, at the surface of thearticle, the metals are in the desired alloy proportion.

In the gold base it is advantageous to leave at least a thin layer in which the proportion of indium is less than 8%., The surface may have any amount of indium over 10% depending upon shown that with increasing proportions of in- 4 Claims. (01. 29-1835) 2 the use to which the product is to be put. In articles for use .as such the indium content should be less than 50% at the surface and preferably about 40%, but when the product is to be used for soldering, the surface may be much higher in indium content and in some cases should consistof unalloyed indium.

In most cases, in the practice of my invention, it is advantageous to use pure indium as a plating, foil, powder, or bath; but it is also feasible, and in some cases may be desirable, to use an alloy which is relatively high in indium as compared with the desired alloying of the surface. Advantageously pure gold may be used, especially as foil or electro-plated onto the"surface of the article to be treated; but less pure gold, e. a, 18 kt. may be used.

Although I am giving herein, a number of ex amples of my invention and am suggesting various alternatives and modifications, it should be understood that these are not intended to be exhaustive nor limiting of the invention, but on the contrary are given for purposes of illustration and with a view to instructing others in the principles of my invention and the practical application thereof, in order thatothers skilled in the art may be enabled to practice the invention and to apply it in numerous forms and with numerous modifications, each as maylbe best adapted to the conditions of a particular use.

In. one example of my invention, I have also found that, by means of this invention, articles which require severe mechanical working can be made of alloys which are incapable of such mechanical working. Thus for example, goldfoil (including even gold leaf) of high indium content gold alloys is made by first forming the gold foil in the usual way, then electro-plating this with indium, e. g., in amount equal to about 30% by weight of the unplated gold foil, which provides the proportion, e. g., about 22 /2%, desired in the final alloy foil; and then heating the plated foil to a temperature above the melting point of indium, but below the melting point of gold and of the desired alloy, e. g., 165 for 2 hours. g

The 22 /2% indium I'l /2% gold forms a eutectic which melts at about491 C. The percentage of indium at this point, however, is not sharply critical and substantially equivalent results are obtained with a variation of about 5% plus or minus.

The'resulting foil, or leaf, may be used as such I aesaeer briquettes or blanks, is placed between surfaces to be joined and held in pressure contact. A strong bond is formed by diffusion which is resistant to temperatures well above those at which the bond is formed. This is particularly important where heat treated parts are to be joined and where brazed joints are required in apparatus which contains parts which would be in danger of fusing, decomposing or cracking at temperatures of fusion of the bonding alloy, especially if the bond itself may be subjected in service to local heating to a range close to that at which such damage might occur. This diffusion welding is more particularly described and claimed in a copending application Serial No.

453,466, filed August 3, 1942.

As another example, a bracelet or watch case may be plated or otherwise surfaced with gold to a thickness of at least 0.0001". Onto this I apply by electroplating, a film of substantially 165 C. for a period of two hours, whereupon it will be found that the surface of the articl is composed of an' alloy of about 40% indium and a 60% gold. This alloy is a whitish gold, composed of small crystals and is very hard and resistant to wear without showing any signs of brittleness, although an alloy 01' the same proportions made en masse by fusion would be too brittle for ordinary uses.

It is an important discovery that if enough gold is present beneath the indium plating the surface will, upon heating at 165 C., tend always to come to a composition of about this same percentage. If a higher percentage of indium is desired, e. 8.. to give a bluer color, the heating may be controlled to a shorter period to restrict diffusion, or more advantageously the amount of gold in the underlying area may be limited so that the proportion of indium by weight is relatively high; Conversely, if a lower percentage of indium in the surface is desired, e. g., fora yellower color, an outer plating of gold may be applied over the indium, or gold and indium may be simultaneously plated onto th surface.

In any case, I.find that the best results are at; tained by leaving a layer of gold below the surface with an indium content not exceeding about 8%. At greater depths below the surface, the

percentage of indium will be correspondingly less.

I claim:

1. An indium-gold surfaced article which com-v prises a surface film composed of an indium-gold alloy the indium being present in proportion not less than 10% and the gold in proportion not less than 50% at the surface. an under layer of gold containing not over 8% of indium and an intermediate layer in which the proportion of indium increases gradually toward the surface.

'2. An indium-gold foil which comprises at opposite surfaces thereof an indium-gold alloy having at least 10% of indium and at least 50% of I gold, a central zone of gold containing not more than 8% of indium and an. intermediate zone in which the percentage of indium increases toward said first-named layers and the percentage of gold increases toward the second-named layer.

3. The method of making an article having its surface composed of an indium-gold alloy of more than 8% indium content which comprises forming the article with a gold surface, plating a layer of indium thereon not more than the weight of the gold in the same area on said article, and heating said article at temperature between C. and C. until the indium has diffused into the gold to such extent as to leave the desired metal composition at the surface, a

4. The method of making an article having its surface composed of an indium-gold alloy of more than 8% indium content which comprises forming the article with a gold surface, applying thereon a surface layer containing in excess of 50% indium, and the indium thereof being not more than the weight of the gold in the same area on said article, and heating said article at temperature between 160 C. and 170 C. until the indium has diffused into the gold to such extent as'to leave the desired alloy composition at the surface.

HAROLD D. ELLSWORTH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATESPATENTS Cooper Dec. 15, 1914 Gray Oct. 28, 1930 Gray May 29, 1934 Taylor Jan. 8, 1935 Kohler Aug, 20,1940 Weiss May 27, 1941 Hobbs Dec. 16, 1941 Hardy Sept. 29, '1942 Queneau Sept. 14,1943 FOREIGN PATENTS Country Date Great Britain Oct. 12, 1931 OTHER REFERENCES Ludwick, Indium and Indium Plating, Art. in

Number I Metal Finishing, vol. 40, Jan. 1942, pp. 13-17 incl.

Kubaschewski et al. Uber die Legierungen des Indiums mit Gold, Art. in Zeitschrift fur Elektrochem, vol. 44, 1938,-pp. 870-877..

Hastings Dec, 30, 1873 Stockmar Jan. 25, 1938 Smart July 7, 1942' 

